Plasma separator ion engine



April 1969 R. N. EDWARDS PLASMA SEPARATOR ION ENGINE Filed April 18, 1962 INVENTOR. 05.27! /1. fan mew United States Patent 3,436,582 PLASMA SEPARATOR ION ENGINE Russell N. Edwards, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Apr. 18, 1962, Ser. No. 188,335 Int. Cl. HOSh 1/00, 3/00 US. Cl. 313-63 2 Claims The present invention relates to a plasma separator ion engine and, more particularly, to an engine from which thrust is obtained by creating ions, accelerating them electrostatically and exhausting them at high velocity.

Electrical space propulsion makes use of engines in which electrical energy is converted into kinetic energy of a propellant to produce thrust. One of the systems used charges the propellant electrically by converting it largely into ions. Electrostatic acceleration is then used to increase the propellant velocity to very high levels to produce thrust. The efiiciency with which propellant may be accelerated electrostatically is dependent upon the ratio of the beam power to the input power which must also include electrical losses for the generation, acceleration and neutralization of the ion beam.

One of the conventional ways in obtaining thrust in this manner is to provide an emitter, which may be an electrically charged surface, to generate ions (commonly produced by surface ionization) which are subsequently accelerated and exhausted to produce thrust. The emitter must be maintained at a high temperature to avoid reduction of its work function by cesium adsorption. The radiant heat loss from the emitter surface is substantial. Therefore, a moderately high current density is required for efficient operation and it is necessary to apply a large extracting field, to overcome space charge forces. In typical engines, the current density desired for eflicient operation requires voltage on the verge of electrical breakdown.

Another approach makes use of very high density plasma generated ions which are extracted immediately by an extremely large electric field. This approach requires unusually well-conditioned (cleaned, polished, etc.) electrodes to avoid electrical breakdown and results in highly critical design parameters.

The literature in this field, as those skilled in it well know, is extremely voluminous. A convenient general reference of relatively recent date is Ionic and Plasma Propulsion for Space Vehicles, G. R. Brewer, M. R. Currie, R. C. Knechtli, Proceedings of the Institute of Radio Engineers, vol. 49, pp. 1789-1821, December 1961, which gives nearly fifty additional references.

The present invention separates the extraction and acceleration process from the ion generation process in such a way that each may be separately optimized. This allows the use of highly efficient, high pressure arcs and high density surface ionization plasma generators in conjunction with low current density accelerating systems which also provide highly efficient operation. This is obtained generally by using a plasma source, rather than an ion source, to eliminate space charge forces in the extraction region.

The main object of the present invention is to provide a plasma separator ion engine which separates the extraction and acceleration processes from the ion generation process so that each may be separately optimized.

A further object is to provide such an engine by which it is possible to maintain constant power output with a varying specific impulse. This is desirable in applications in which available electric power, rather than available working substance, is the limitation upon thrust. Well known laws of mechanics teach that thrust in a reaction type of engine is proportional to mass of working substance discharged per unit of time multiplied by the velocity with which it is discharged; but the power imparted to the working substance is proportional to the mass or working substance discharged per unit of time multiplied by the square of the velocity with which it is discharged. Thus, it requires less power to produce a given thrust by discharging more mass at lower velocity; or, expressing the same relation in another way, for a given available power, more thrust may be produced by discharging a larger amount of working substance at lower velocity.

Another object of the invention is to disclose a typical plasma source that may be used with the combination plasma separator ion engine of the invention and forms part of a typical combination.

Briefly stated, the invention provides a plasma separator ion engine having a high density neutral plasma source to create a neutral plasma which is then expanded preferably in a beam into a low density area downstream of the source. A separator system is provided in the low density area to extract ions from the plasma at that point and neutralizer means are provided downstream of the separator to emit electrons into the plasma and give a high velocity beam for thrust.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

FIGURE 1 is a schematic outline of the plasma separator ion engine of the present invention;

FIGURE 2 is a partial cross sectional view of a typical neutral plasma source that may be used in the combination of FIGURE 1; and

FIGURE 3 is a partial perspective showing typical accelerator-decelerator and neutralizing means used in the downstream low density area and is an enlargement of the area shown at A in FIGURE 1.

Referring first to FIGURE 1, the general concept of the present invention is shown wherein there is provided a neutral plasma source 10 that provides a high efiiciency high density plasma. The plasma generated in 10 is expanded, preferably by controlled nozzle means 11, to a sufiicient velocity to allow its control in a well defined expansion beam 12 which occupies a space in which the plasma beam expands from nozzle 11 into a large low density area downstream at 13. At that point, a plasma separator-accelerator system, generally indicated .at 14, extracts the ions from the low density plasma by an electrostatic field and accelerates them to the required velocities in a plasma beam 15. The separator-accelerator 14 may be shaped as shown in FIGURE 1 to produce a converging plasma beam 15. In addition, the plasma source 10 and/ or separator-accelerator 14 are movable relative to one another and FIGURE 1 illustrates source 10 as being movable by any suitable means generally indicated at 16. The purpose of the movement of source 10 and separator-accelerator 14 relative to one another is to allow operation over separator areas corresponding to a large, low density, low specific impulse beam at maximum separation or a small, high density, high specific impulse beam of comparable power level when the separation is less. This provides a variable specific impulse with a constant power output which is of definite advantage, since the power level varies as the fifth power of the specific impulse with fixed engine configuration.

While a cesium plasma will be discussed for illustrative purposes only it should be understood that the invention is applicable to many materials that may be used to generate plasma and tantalum with cesium is merely used as an example but any suitable high work function refractory metal may be used with an appropriate alkali metal.

Reference is now made to FIGURE 2 wherein a typical neutral plasma source is illustrated for the purpose of further discussion with respect to the invention. In this figure, a current conducting rod 17 is provided and this is surrounded by a conductive casing or shell 18 to define an annulus 19 therebetween. A tantalum screen 20 is connected to and extends from rod 17 to complete the current carrying path as indicated by the plus and minus signs. Thus, screen 20 is heated by the electric current and cesium vapor is introduced through the annulus 19 as indicated by the arrows to pass through the screen (which may be heated to a temperature of about 2400 degrees Kelvin, whence it is a copious electron emitter) the cesium vapor is ionized on passing through the screen to result in an equal mix of cesium ions and electrons which is a high density neutral plasma 21. Because the plasma is neutral it is free to move as a neutral gas and there are no space charge forces acting on the cesium ions (which are positively charged) to force them back to their source; resulting in the neutral plasma moving downstream by pressure force and decreasing in density as it flows downstream.

Returning now to FIGURE 1, this is the high density neutral plasma in beam 12 that expands downstream from nozzle 11. In the case just described, it is, of course, a cesium plasma. At a suitable point in the downstream area where the plasma density is low the separator-accelerator 14 is introduced.

Plasma separator-accelerator 14 may comprise several elements as shown more in detail in FIGURE 3. At a suitable location for separator-accelerator 14 the density of cesium ions on the surface of the plasma in beam 12 is comparable to the density of the cesium ions on the surface of the emitter in the conventional ion emitter source. At this point it is desirable to introduce the separatoraccelerator to separate the ions from the neutral plasma. The separator-accelerator extracts the ions, accelerates them, after which electrons are added to neutralize them yielding a space charge free plasma beam to obtain thrust. Thus, the details of separator-accelerator 14, as shown in FIGURE 3, may comprise any suitable separator elements 22 which may be merely knife edges or the like followed by acceleration elements 23 through which the ions are focused by separator elements 22 and accelerated by means of an electrostatic field or accelerator elements 23. These, comprising the separator-accelerator are, in turn, followed by decelerating elements 24 which again create an electrostatic field to obtain the final voltage desired. These are then followed downstream by a neutralizer element 25 which may employ a trapping electrode to retain the electrons in a definite region to mix with the ions coming downstream. This prevents the return flow of the positively charged ions upstream so that a high velocity neutral plasma is thus directed downstream to produce thrust. All of these elements just described may conveniently be placed, as shown in FIG- URE 1, in a series of screens transversely of beam 12 and FIGURE 3 is a greatly enlarged view of a small element of separator-accelerator 14 at point A in FIGURE 1. The overall screen arrangement may then be curved as shown in FIGURE 1 to mechanically stabilize the separatoraccelerator to focus the plasma beam into a converging shape shown at 15 to produce the required thrust. In addition, separator elements 22 may serve as focusing electrodes to direct the plasma beam through the accelerating electrodes or elements 23, instead of on them, as clearly shown in FIGURE 3.

Separator-accelerator 14 thus establishes the boundary of the low density expanded plasma at the potential of of the plasma source. The potential difference between the separator 22 and the accelerator 23 then provides the electrostatic field to accelerate the ions into the exit region. In addition, the separator accelerator system 14 makes use of suitable potential distributions to ensure low interception of accelerated ions and the exit ions are decelerated in the region between the accelerator and the neutralizer to provide a region into which electrons can be introduced by element 25 without fear of their acceleration back to the separator. Brewer says, pages 1797- 1798, Essentially all modern cesium ion engines operate in this so-called accel.-decel. mode. In this mode, the final exhaust of the propellant taeks place at or near the potential V corresponding to the final or decelerator electrode, but between this electrode and the ionizer the accelerator electrode is arranged at potential V,,. The fraction V /V is called the acceL-decel. ratio; typical values are approximately 2 to 10. The neutralized plasma beam then leaves the region at high velocity to provide the required thrust. By the separation of the extraction and acceleration process from the ion generation process it is possible that each may be separately optimized with the advantages heretofore mentioned.

While I have hereinbefore described a preferred form of my invention, obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. A plasma separator ion engine comprising,

a high density neutral plasma source including a central current conductive rod,

a conductive shell spaced from and surrounding said rod to define an annulus therebetween,

a perforate screen of refractory metal of high work function heated by electric current at a potential less than the electrical breakdown potential of an alkali metal vapor to a temperature at which it emits significant quantities of electrons connected to and extending from said rod and disposed within said shell,

and means to direct said alkali metal vapor through said annulus and screen to produce by contact ionization an equal density of alkali metal ions and electrons for a balanced neutral plasma,

means to expand the neutral plasma in a beam into a low density area downstream of the source,

a separator system in said low density area to extract ions from the plasma,

and neutralizer means downstream of the separator to emit electrons into said ions and yield a high velocity plasma beam.

2. In a plasma separator ion engine comprising a high density neutral plasma source,

means to expand the neutral plasma into a low density area downstream of the source,

a separator system in said low density area to extract ions from the plasma,

and neutralizer means downstream of the separator to emit electrons into said ions and yield a space charge free high velocity plasma beam,

the improvement comprised by the modification that the said plasma source and the said separator system are relatively movable to one another to maintain a constant power output with varying specific impulse.

References Cited UNITED STATES PATENTS 3,020,431 2/1962 Martina 3 l 3--230 X 3,050,652 8/1962 Baldwin 3 l363 3,279,176 10/1966 Boden 313-63 X JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

U.S. Cl. X.R. 60202; 313-231 

1. A PLASMA SEPARATOR ION ENGINE COMPRISING, A HIGH DENSITY NEUTRAL PLASMA SOURCE INCLUDING A CENTRAL CURRENT CONDUCTIVE ROD, A CONDUCTIVE SHELL SPACED FROM AND SURROUNDING SAID ROD TO DEFINE AN ANNULUS THEREBETWEEN, A PERFORATE SCREEN OF REFRACTORY METAL OF HIGH WORK FUNCTION HEATED BY ELECTRIC CURRENT AT A POTENTIAL LESS THAN THE ELECTRICAL BREAKDOWN POTENTIAL OF AN ALKALI METAL VAPOR TO A TEMPERATURE AT WHICH IT EMITS SIGNIFICANT QUANTITIES OF ELECTRONS CONNECTED TO AND EXTENDING FROM SAID ROD AND DISPOSED WITHIN SAID SHELL, AND MEANS TO DIRECT SAID ALKALI METAL VAPOR THROUGH SAID ANNULUS AND SCREEN TO PRODUCE BY CONTACT IONIZATION AN EQUAL DENSITY OF ALKALI METAL IONS AND ELECTRONS FOR A BALANCED NEUTRAL PLASMA, MEANS TO EXPAND THE NEUTRAL PLASMA IN A BEAM INTO A LOW DENSITY AREA DOWNSTREAM OF THE SOURCE, A SEPARATOR SYSTEM IN SAID LOW DENSITY AREA TO EXTRACT IONS FROM THE PLASMA, AND NEUTRALIZER MEANS DOWNSTREAM OF THE SEPARATOR TO EMIT ELECTRONS INTO SAID IONS AND YIELD A HIGH VELOCITY PLASMA BEAM. 